Dutch scientists enrolled more than 250 children. When the children were 4 weeks old, their body dimensions and lung function were measured. At the same time, their parents completed questionnaires about such factors as smoking during pregnancy.

When the children were 5, the researchers used ultrasound to measure the thickness and flexibility of their carotid arteries, large blood vessels in the neck that supply blood to the brain. They also collected updated smoking information from their parents.

The walls of the carotid arteries in 5-year-olds whose mothers had smoked throughout pregnancy were about 19 microns thicker -- about one to two times the thickness of a piece of cassette tape -- and 15% stiffer than those whose mothers had not smoked.

If both parents smoked while they were in the womb, the children’s carotid arteries were nearly 28 microns thicker and 21% stiffer than those of children whose parents didn’t smoke during pregnancy. These changes may indicate damage to blood vessels that may affect their function, the study authors suggest.

The scientists did not find an effect if only the father smoked during the pregnancy, or if the mother hadn’t started smoking until after giving birth.

Smoking During Pregnancy vs. After

“The challenge there was to show that it was really smoking in pregnancy” and not exposure to cigarette smoke afterward, researcher Cuno Uiterwaal, MD, PhD, says in an email to WebMD. “To further explore that, we did this study.”

Uiterwaal, an associate professor of clinical epidemiology at the Julius Center for Health Sciences and Primary Care at the University Medical Center Utrecht, Netherlands, says, “with our findings, we think that smoking in pregnancy does play an independent role, although we know that exposure of children to [secondhand] smoke is damaging in many areas.”

“Smoking in pregnancy is bad for many reasons for the mother and certainly also for the child,” Uiterwaal says. “Our findings may well be another argument to quit during pregnancy. Many women do quit as soon as they know they’re pregnant, but not all do.”